Maximising biogas in anaerobic digestion by using engine waste heat for thermal hydrolysis pre-treatment of sludge


Dublin's Ringsend WWTP was designed to serve a population of approximately 1.2 million pe with a sludge production of 37,000 dry tonnes per year after upgrading to full secondary treatment. Several technical solutions were put forward as part of a Design, Build, Finance and Operate (DBFO) competition, with the chosen solution being a proposal by Black and Veatch for a combination of Sequencing Batch Reactor (SBR) technology and anaerobic digestion with Cambi thermal hydrolysis pre-treatment (THP). The THP plant was built by Cambi and handed over to B&V in 2002. The plant is now operated by Celtic Anglian Water. A 2004 performance report has just been carried out on the mass and energy balance of the plant following 2 years of operation and is detailed in the paper.

Celtic Anglian Water were selected in 2004 for the Sustainable Energy Award out of field of 250 entrants in Ireland. The award recognises a commitment to energy management. Dublin’s Ringsend Wastewater Treatment Works was also the winner of the 2004 CIWEM Ken Roberts Award for Technical Innovation in the Water Industry.

Conventional design of anaerobic digestion would have called for about 36,000 m3 of digestion capacity and at a VS destruction of 50% VS. The Dublin digestion plant has a pre-treatment plant using the Cambi thermal hydrolysis process (THP). The process enables digestion at very high dry solids feed and low hydraulic retention time. The plant
was built with 3 digesters of 4,250 m3 each and is fed with hydrolysed sludge at 11% DS. The net effect is that the plant can produce 3.5m3/day of biogas per m3 of digester capacity, which is 350% of the normal situation.

There are 4 no. 1 MW Jenbacher engines operating mainly on biogas. Each pair of engines is fitted with a waste heat boiler with a capacity of one tonne steam per hour. These boilers have sufficient capacity to provide 70% of the steam required for the THP, which in turn provides all the heat for the subsequent digestion in the form of hydrolysed feed. There are also main biogas boilers for top up steam and other uses of the biogas including thermal oxidation of concentrated odours.

This ability to get much more out of a digester has consequences for upgrading existing digestion projects where digester capacity is limited or where a plant has limited space for digestion expansion. It also means that digestion plants can be converted into regional facilities in order to maximise power production from imported sludges from smaller plants.

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